Hurricanes: What's Next? is part of the University of Michigan's Teach-Out series. For more information, visit teachout.org.
The 2017 Atlantic Hurricane season produced several incredibly destructive storms, and raised many questions. As we enter the 2018 hurricane season, we will re-explore the following questions: What drives a hurricane? How accurate are hurricane models? How do authorities prepare for hurricanes and, when destructive events like Hurricanes Harvey, Irma, and Maria happen, how do we respond? Was 2017's hurricane season a fluke, or should we start planning for similar storms to appear more frequently? In this Teach-Out, we will explore the science of hurricanes, hurricane forecasting and monitoring, and with what confidence can we attribute these storms to a warming ocean.
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Hurricanes: What's Next?

The 2017 Atlantic Hurricane season produced several incredibly destructive storms, and raised many questions. As we enter the 2018 hurricane season, we will re-explore the following questions: What drives a hurricane? How accurate are hurricane models? How do authorities prepare for hurricanes and, when destructive events like Hurricanes Harvey, Irma, and Maria happen, how do we respond? Was 2017's hurricane season a fluke, or should we start planning for similar storms to appear more frequently? In this Teach-Out, we will explore the science of hurricanes, hurricane forecasting and monitoring, and with what confidence can we attribute these storms to a warming ocean.

Conoce a los instructores

Perry Samson

Arthur F Thurnau Professor, Professor of Climate and Space Sciences and Engineering, College of Engineering and Professor of Information, School of InformationCollege of Engineering; School of Information

Welcome to the Teach-Out. This is a Teach-Out on hurricanes.

As part of this Teach-Out,

we started in 2017 talking about the large number

of outbreaks of hurricanes that year, very destructive hurricanes.

This time, we're going to add to it some new information,

where we interview people who are experts in the field,

who are actually in the field,

we're in Puerto Rico,

for example, and their experiences in the field and how that affected,

how their efforts are playing a role in helping people recover in those areas.

We will also be talking some about new information about how climate change might be

impacting hurricane strength, duration, and speed.

Some new information about how experts are trying to quantify

just how much could climate change actually be

changing the strength and intensity of these hurricanes.

Now, I teach a course here at the University of Michigan in extreme weather.

This course allows me to interact with the students and

have them ask me questions that we can use as part of this exercise.

So here's a question from one of my students.

I mean, this is a basic question.

I never knew how hurricanes formed.

So, how did they form? How did they start off?

That is a very basic question.

So, in the part of this Teach-Out,

we'll start with that discussion.

One way to do that is take a look at a hurricane.

Here's a fantastic video from NASA,

in which we can see the characteristic features of hurricane.

The eye of the hurricane. The center here.

We can see all the way down to the ocean surface.

Around is the eyewall.

You can see there some strong storms.

Now, if you stare at this long enough,

there's many features going on here which are important to the formation of a hurricane.

One of them, you can see sort of a spiral inside the eye itself,

and that really represents something called hot towers.

We're going to talk about what hot towers are and

their role in the formation of the storm.

But the storm is causing

large cumulonimbus clouds rising up 10 miles or so into the atmosphere,

and as the air rises up,

it gets to the top of the troposphere,

spreads out at the top and you kind of see in here

also the outflow going out on hurricanes.

So, at the surface, the air is flowing in

a counterclockwise way into the center of the storm.

At the top of the atmosphere,

it's spiraling out in the opposite direction.

So you have both going on very characteristics of a strong hurricane.

Now, the hurricanes are made from processes wholly different from what we have,

say in the Midwest.

Here in Michigan, we have many storms,

large precipitation events and mostly those are called synoptic scale storms.

On that scale, they're produced by different mechanism,

where the differences in temperatures between

North and South produce gradients in temperature,

which create gradients in pressure in

the atmosphere and those drive both the winds and also

then drive other factors which are going to cause

storms in the more temperate parts of the country.

But hurricanes are unusual beast in that,

they are formed entirely from the release of latent heat.

That is, as water vapor is rising inside the storm,

it's going to produce Hurricane Irma and Hurricane Jose in 2017.

We get some sense of the scale of these storms.

They are, obviously, perusing large amounts of precipitation.

They're also producing very strong winds.

Now, how did we get to a storm that's of

that scale producing that level of precipitation?

Here in the cross-section,

you can see from NASA radar,

a picture of where precipitation is inside the hurricane.

It's not evenly distributed.

It's very strongly there near the center of the storm.

These hot towers and strong thunderstorms here on the eye of

the hurricane then releasing great amounts of heat.

Air is flowing from the surface into the clouds.

Inside the clouds, they're condensing into liquid water.

Liquid water goes through a process of condensation.

The condensation is releasing heat at a rate of

600 calories for every gram of water being condensed.

That released heat means the inside of the cloud is actually going to

be a little bit warmer than the air outside the cloud,

hence you get convection,

hence you get this upward motion inside the cloud.

That's what's going to carry the moisture from the surface higher into the atmosphere.

As it does that,

it's going to produce a low pressure.

Basically, you're moving mass from that column throwing it to the top of the atmosphere,

spreading out producing a lower pressure,

reducing amount of mass in the center of the storm via a change of pressure in the storm.

So you now have what we call a pressure

gradient from outside the storm to inside the storm,

that's going to produce a stronger wind.

We'll talk about all this in a moment.

But that stronger wind means you're going to pick up a lot more moisture,

waves, splashing and such,

more evaporation at the surface,

more moisture is available.

More moisture condensing produces a stronger hot tower,

and more heat is released,

and the storm is just driven entirely

then by how much moisture is available in the atmosphere.

That's going to come back. We're going to talk about that because if,

in fact, the climate is warming,

then potentially the surface of the ocean is going to be

warmer and there'll be more potential evaporation from the ocean,

there we have some concerns that it's going to lead to yet more convection in the future.

Now, this particular storm was about 425 miles wide.

It's about 10 miles high.

So that's a large volume of air in which this condensation is occurring.

We're going to be measuring then the total energy these storms develop.

Energy inside the storm is going to be measured by how much wind speed it's creating.

The wind speed is by itself a good measure of how strong the storm is,

but we have to remember the power of the wind.

The power of the wind goes up as the cube of the wind speed.

So, if we have a wind speed,

say 75 miles an hour,

and one storm, and a wind speed of 150 miles an hour in another storm,

so you've doubled the wind speed,

what you're doing there is increasing the power of the storm by a factor of eight,

two times two times two, in the process.

So, small changes in wind speed produce great changes in the power of the storm.

We're trying to quantify how much power each storm represents.

Damage from storms comes from a number of processes.

One is, obviously, wind speed but the second is going to be precipitation.

As we'll talk about in a moment,

how we classify storms at the moment is largely to do with wind speed,

but we also understand that many of

these storms now are producing huge amounts of precipitation,

which also cause a great amount of damage.

As we have saw in 2017 in

the Hurricane Harvey in the Gulf States and Texas, in particular.

In Texas, some 50 inches of rain fell in 24 hours in some parts of Texas.

It's unbelievable amount of precipitation here in the Midwest,

that's basically up to my neck,

and precipitation or dropping a good portion of

a swimming pool on the homes in that area.

So that precipitation has to drain into the soil and some of them can't drain

fast enough and we wind up with flooding and then,

the wind itself was also going to move things around and caused great destruction.

So, as part of our Teach-Out here,

we're going to think about how wind speed and the precipitation play roles in

the strong power and destructive capability of a hurricane.

Now, hurricanes don't just happen.

They go through stages.

We're going to talk about the whole process,

the evolution of a hurricane from its inception through.

Strong Category Five hurricane.

But to get to hurricane stage,

we'll begin often with simply what we call a tropical cyclone.

Often out of the ocean,

these tropical cyclones are

more or less just waves in the atmosphere and

these waves are causing areas of cloudiness.

Again, as we go through this,

we're going to discuss how those waves of cloudiness then begin to

organize the clouds so they kind of work in concert to release the latent heat.

Once you've done that, now you sort of get a center of heat and a core

around which the air can begin to circulate taking us into a tropical disturbance,

an early stage of a tropical cyclone.

Tropical depression means now the storm is organized,

that is, it has a center and is beginning to have a rotation around the center.

It's important if you look at a map of where hurricanes occur.

You're going to see that many hurricanes

occur in the Northern Hemisphere and the Southern Hemisphere,

but none occur at or near the Equator.

At the Equator they get plenty of thunderstorms but they don't

organize into rotating storms because there is no Coriolis force.

We're going to need also a Coriolis force to help these storms begin to rotate.

We'll talk about how that happens.

You get to a tropical depression in wind speeds inside the storm

on the order of 39 miles an hour or less,

it's certainly a nasty storm but not enough to cause great deals of damage.

Tropical storm is now in the new range where it's rotating.

It's not a nice pattern we're going to see in a hurricane.

It's more of a circular organization of clouds.

It's going to be more of a scattering of clouds.

It's sort of a birthing stage,

if you will, of the hurricane.

At that point, the wind speeds are going to be somewhere

between 39 miles an hour and up to 74 miles an hour.

Now, when you hit 74 miles an hour,

you then cross the threshold at which point we start calling it a hurricane.

At 74 miles an hour,

this is an entry-level hurricane.

We measure hurricanes based on the Saffir-Simpson scale,

a scale of one to five in terms of intensity.

But you can see on this slide that the intensity is entirely has to do with wind speed.

This Category One is somewhere around 75 miles an hour.

Category Five is around 150 miles an hour.

So, again, between Category One and Category Five,

the power of this storm is going to be going up as

a factor of eight because the wind speed cubed.

Now, in 2017, we've had a large number of storms of hurricanes happening, and in fact,

three of the storms that happened in 2017,

now are in the top five of most costly storms or hurricanes in history.

So, 2017 stands out as a particularly unique year in terms of

hurricane destruction. Why is it?

Why 2017?

Why is it some years we get a lot of hurricanes?

2005, we got a lot of hurricanes.

2017, we got a lot of hurricanes.

Other years, none or very few.

So what happens?

Why are some years different?

That's why we're going to talk about the details of

why there's such variation from year to year.

If you are in a location though and you hear either of these two words,

you must take note,

Hurricane Watch means the conditions are right.

The storm is out there over the ocean.

The storm is going to come onshore and there's

a probability that storm is going to be coming to your area.

So, this is the point at which you want to start making plans for what will you do.

How do you bolt up the windows?

Is there some place you can go,

like back to Michigan where we don't get any storms like this.

Is it possible for you to leave?

If you are in Puerto Rico,

what would you do when a Hurricane Watch happens?

What are your options?

It is important to think about.

A Hurricane Warning means a storm is

imminent and the probability of hitting your town is extremely large.

At that point, you should have taken the action you need to get, if you can,

to remove yourself from that area if you're near the water,

because one of the big concerns is going to be a storm surge where the ocean itself is

brought on the shore and the flooding is going to cause the damage in your area.

If you can get to high ground and maybe that's safer.

If you think about,

listen to the people who are from FEMA or whoever gives

you the advice about what you might do next to protect yourself.

But this is a serious situation when Hurricane Warning is occurring in your area.

Let's walk through each of these five categories of hurricanes.

We have some data from the National Weather Service showing us

that each of these hurricanes has its own strength.

Hurricane Category One, 74 miles an hour is the entry fee into hurricane status.

At that point, these are storms which are serious but they're not going to cause

maybe as much damage as some of the more bigger storms here.

Primary damage to unanchored mobile homes, shrubbery, and trees.

There will be damage but it's not generally going to be as life-threatening.

That said, every year in my class

I choose a student and we take the student and stick them in

the wind tunnel here at the University of Michigan and turn it up to a hurricane forest

about 75 miles an hour and film the student standing inside their thing.

Now, the student, please understand, is roped in.

Still a tad unnerving because you're inside

a dark chamber and there's a high winds and

you know there's some fans somewhere behind you.

You don't want to fall down into the wind tunnel.

But even standing there,

it's really hard to stand up even though you're roped in.

It's really hard to stand up at 75 mile an hour winds.

If you turn your face sideways,

due to the Bernoulli Effect,

the air is kind of blowing past your mouth.

It makes it really hard even to breathe at 75.

So, an entry-level hurricane is still

a significant windstorm and is to be taken quite seriously.

Category Two Hurricanes.

Now we're getting into the issue of storm surge.

We're going to talk about storm surges,

such as the water being blown on land by the winds coming onshore.

You've got both the wind speeds blowing the air onshore.

We got the lower pressure inside the hurricane causing

the water level to be a little bit higher to begin with.

On top of that if it comes onshore at high tide,

you've got the addition of the effect of

the moon there on how much a storm surge is going to happen in your area.

This can cause a great deal of damage.

Remember, on top of the wind speeds goes,

there's going to be how much precipitation.

Are these storms moving slowly?

If they're moving slowly, you might be under the precipitation bands for a long time,

and that in fact we will talk about later,

is one of our concerns for the future as it's possible that as climate warms,

these storms may not be more of them but they

might move slower and they might have more energy in them,

which could lead to far more precipitation.

We'll talk about why. Category Three Hurricanes.

Now up to 130 miles an hour.

Low-lying escape routes they're might be cut off because

of flooding by either of the precipitation or,

if not, the storm surge.

Category Three, again, is the place,

a time where you probably don't want to be in

your home if you're anywhere near the coastline.

Category Four.

We're up to 155 miles an hour at this point.

There's going to be a failure of buildings.

There's going to be strong winds.

Obviously, there's going to be rising water,

and the great damage again is largely a storm surge.

The only thing worse than a Category Four Hurricane,

of course, is the Category Five,

and at this point, you have a storm which is wind speeds in excess of 155 miles an hour.

Eight times more power in that wind in a Category Five.

This is precisely the kind of storm which hit Puerto Rico.

This storm coming onshore did

both the high wind speed damage and the large amounts of precipitation.

Very, very deadly storm hitting Puerto Rico.

The storm coming onshore.

Hurricane Harvey, was not a Category Five.

It was the lower category but it caused its damage because of the precipitation.

So, storm surge, flooding from precipitation and the high wind speeds all combine,

each of them can produce their own level of damage.

Well, this is a teacher, and as a teacher,

we were encouraging you to participate by jumping into the forum

and asking your own questions or commenting on things that you heard.

As part of this, we're also going to have videos about the formation of hurricanes,

the physics of that.

We're going to talk about whether and to

what degree climate change might be playing a role in hurricane formation.

Is it going to affect their strength or their duration in time or the speed?

Also, we're going to talk to experts about who have been in the field,